TRANSACTIONS OF THE SECTIONS. 17 



e e are the two eyes of the observer placed before two 

 plane mirrors, incluied to each other at an angle of 

 90°; the axes of the eyes converge to a point c; the 

 pictures p p are so placed on sliding panels, that 

 their reflected images may be adjusted to appear at 

 the place of convergence of the optic axis ; it is ob- 

 p' vious, then, that the pictures on the retinae will be 

 precisely the same as if they proceeded from a real object placed at c. 

 In this manner may solid geometrical forms, crystals, flowers, busts, 

 architectural models, &c. be represented with perfect fidelity, as if the 

 objects themselves were before the eyes. The law of visible direction, 

 which is universally true, for all cases of monocular vision, may. Pro- 

 fessor Wheatstone stated, be extended to binocular vision, by the 

 following rule : That every point of an object of three dimensions is 

 seen at the intersection of the two lines of visible direction, in which 

 that point is seen by each eye singly. 



Observations on Stars and Nebulce at the Cape of Good Hope. 

 By Sir John F. W. Herschel, Bart. F.R.S., ^c. 



The notice of these observations laid before the Section reduced 

 itself to the following heads : 



Reduced Observations of 1232 Nebulae and clusters of Stars, made 

 in the years 1834, 5, 6, 7, 8, at the Cape of Good Hope, with the 

 twenty-feet Reflector. 



Reduced Observations of 1192 Double Stars of the Southern He- 

 misphere. 



Micrometrical Measures of 407 principal Double Stars of the South- 

 ern Hemisphere, made at the Cape of Good Hope, with a seven-feet 

 Achromatic Equatorial Telescope. 



A List of the Approximate Places of fifteen Planetary and Annular 

 Nebulae of the Southern Hemisphere, discovered with the twenty-feet 

 Reflector. 



Drawings illustrative of the Appearance and Structure of three 

 principal Nebulae in the Southern Hemisphere. 



The observations in the first two of these communications form parts 

 of two catalogues of southern nebulae and double stars respectively, 

 which comprehend the chief results of the author's astronomical obser- 

 vations at the Cape. They are complete only as far as the first nine 

 hours in right ascension. In ths other hours, only a few of the objects 

 which occur are added, being the results of a partial and very incom- 

 plete reduction of the observations in those hours. Sir John Herschel 

 considered it probable, that when the reduction of his observations 

 shall enable him to complete these catalogues, the total number of ob- 

 jects contained in them will be nearly doubled. The first catalogue 

 contains all the nebulae and clusters comprised in the two Magellanic 

 clouds, which are very . numerous. Each reduced observation ex- 

 presses the mean right ascension and north polar distance of the 



VOL. vn. 1838. c 



